In one embodiment, an electrical power storage system using hydrogen includes a power generation unit generating power using hydrogen and oxidant gas and an electrolysis unit electrolyzing steam. The electrical power storage system includes a hydrogen storage unit storing hydrogen generated by the electrolysis and supplying the hydrogen to the power generation unit during power generation, a high-temperature heat storage unit storing high temperature heat generated accompanying the power generation and supplying the heat to the electrolysis unit during the electrolysis, and a low-temperature heat storage unit storing low-temperature heat, which is exchanged in the high-temperature heat storage unit and generating with this heat the steam supplied to the electrolysis unit.